Rethinking the Process of Vision

(I am posting this Comment in very preliminary form..will workmore on it shortly / GCH)

From the Albert Rose reference discussed previously (“VISION HUMAN AND ELECTRONIC”, Plenum Press) the first few lines from Chapter I are worth quoting:

“It would be difficult to find a more cogent confrontation between physics and biology than in the visual process. …”

(Also, a quote that I have come upon (that will certainly be disputed as outdated….but?) by the English physicist Ernest Rutherford who is credited as the discoverer of nuclear energy: “All science is either physics or stamp collecting.”)

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I have come to believe that answering the never explained question as to how the eye detects light at the quantum limit is the most fundamental question that can be asked of vision science. It’s answer will open up entirely new lines of thought about vision.

Again quoting Rose (p.29 of the above reference):

“In the scheme of evolution, vision has an almost unique role. One can conceive, for example, that further evolutionary development might lead to a larger brain capacity, a more involved nervous system, or a variety of enhancements of current functions. It is not conceivable that the sensitivity of the visual process can be significantly enhanced. The visual process is at an absolute terminal point in the evoutionary chain….”.

I think that the meaning of the term “quantum limit” should be obvious to most. It implies the specific detection and registration in the visual chain of individual particles of light - photons.

(NOTE: I would substitute the term “quantized interaction” for “photon”. As shown in this work the eye actually evolved to detect light as the wave of classical physics adjacent to quantized electrons (the absorbing mass or retinal receptor). My term does not in any way change the meaning of the interaction - it can be variously termed as being “quantized” or “wave defined”. I will continue to use “photon” however and the reader will understand).

To be clear, Rose recounts the many references documenting that the eye is able to sense individual photons renging from two to a few hundreds in number -remembering that each detection is an individual event. I do not believe that there can be any doubt about this.

Rose uses the term to “count” individual photons. Again quoting from p.2:

“The quantum character of light is a hard constraint. Nature could, in a physical sense, do no more and, in a survival sense, do no less than devise a photon counter…..” (emphasis mine).

Now, how does the eye manage to accomplish this? The term “counter” implies segregation of single photon interaction “events” in time, i.e., that they are distinctly separated from each other in the detection process.

It seems appropriate to recycle here some prose that I had written in my Comment of 8/08/06:

“Rose, following from his background in electronics (he was the inventor of the Vidicon tube that had a major impact on the development of television!), led him to propose that an amplification (or electronic ‘gain’) of a million or a factor of 10>6 must somehow follow the original photon interaction signaland be operative in thebiological eye. This is one way to express the system requirement for single photon detection but there are others. Rose could provide no idea as to how the biological eye might accomplish such amplification.

I would note that the limits of contemporary photonic technology allow detection (and counting) of single photons by reducing the temperature of the detector to a few degrees above absolute zero or, at room temperature, by using the electronic gain induced by thousands of volts (as Rose was really proposing) in a vacuum photomultplier tube. The eye performs this single photon detecton function in a biological structure above room temperature - at body temperature!

In photonic technology any signal detection process involves two factors – the inherent (or original) signal level balanced against the random electronic noise that might interfere with (or obscure) that signal level in this time frame in the overall detection system. One must achieve an adequate ‘signal-to-noise ratio’ for unambiguous detection of an event. Accepting the original signal level and reducing whatever noise might be present in that time frame is another strategy for low level signal such as single photon detection.

First, what constitutes the ‘fundamental signal’ when a light wave interacts with an ‘antenna’ absorbing site on the retina as I propose. Such centers (or ‘devices’) are comprised of two adjacent receptors (or more precisely, the quantum- confined electron spaces that the receptors provide) and the wavelength-defining space between them. Light interacts as the wave of classical physics in this central space imparting a different amount of energy to each receptor (with the difference a function of the direction that the light ray entered the device) The energy thus imparted then mechanically/electrically effects an isomeric transition of the retinal molecules contained within each receptor.This then constitutes the initial ‘signal’ that must subsequently processed to provide the visual image.

So what constitutes this most fundamental light detecting devic? I have diagrammed such a deice on my web page (“Important Material”, “Diagram of the Fundamental….). First, the device is is extremely small in consonance with speed of response requirements having a cross section smaller than light wavelength and length the dimension of the receptor outer segments (~50 microns). The time response of any electronic device is dependent upon its dimensions. This is the reason why microcircuitry technology is always seeking smaller ‘feature size’ (i.e., smaller devices) to satisfy the ever present need for increased speed. The charge stopping time (or the time when the signal is ‘there’) I estimate for a device of this dimension to be of the order of 10>-12 seconds (or a picosecond). This is an important number and note that it has nothing to do with subsequent, much slower, “biological signal processing” or transduction time in sub-retinal circuitry or in transit to the brain.

Then, the retina is composed of an array of these very fast devices. I would contend that the initial (and complete) information to form the visual image is present on the retina in some time approximating picosecond ( 10>-12 second) time.

Now back to the subject of ‘signal to noise’ ratio:

Every radiation detection system has an inherent ‘time constant’ that can be considered a ‘time window’ that is ‘left open’ long enough for the signal to be recognized as such and, for example, be electronically transferable to an ‘amplification stage’ for further processing. One property of this ‘time window’ is that electronic noise in the system (i.e., random electron events coming in time) are integrated. The longer the time window is left open the higher the noise that is accumulated within the window. This ultimately constitutes the noise level that obscures the signal. The fundamental rule is therefore that one wants the overall act of detection to be accomplished in as short a time as possible, ideally in a time approaching the ‘signal-is-there’ time

A number of us published a paper on the subject of this optimization in 1976

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Tove,P.A., Cho,Z.H., Huth,G.C., “The Importance of the Time Scale in Radiation Detection Exemplified by Comparing Conventional and Avalanche Semiconductor Detectors”, Physica Scripta, 13, 83-92, (1976).
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I will not go into details of this work here but only to note that the relevant time constant of detection systems of the time was in the microsecond (10>-6 sec.) range. Thus the ‘time window’ was left open for some six orders of magnitude longer than desirable accumulating noise and raising the level of signal that could be detected.

Now to the eye. Validating the 1935 retinal measurements of Osterberg, I have shown that the retina forms the focal or Fourier plane of the converging lens system of the eye. There really can be no doubt about this. It is a property of this plane, as Feynman so cogently explained, that light rays are brought into ‘time convergence’ at this point with light rays that pass through the center of the lens being slowed to allow refracted rays from the thinner outer part of the lens to ‘catch up’. I have written about this elsewhere on this page.

The central fovea of the retina can be used as the prime example with all light rays entering the eye brought to such a time convergence at this point. I have written that such time ‘convergence’ might be defined as ‘zero time’ (or as near as quantum limitations will allow) or what might be thought of as the ‘the absence of time’ or the ‘instant’ of time. Thus the visual ‘signal’ comes as close to the optimum time described above as one might imagine.

The light detection devices of the retina are thus seen to be consistent with the time requirements of Fourier plane imaging so, in fact, the entire content of information necessary to form the visual image is ‘there’ in picosecond time.

I believe that the invocation of the picosecond time domain answers the question as to how the eye detects single quantum events. It is not that the signal is somehow amplified but rather that the contravening noise level is reduced.

With the above insight as to the time regime, it occurs to me that the terminology of ‘detecting single quantum events’ is not strictly correct. This capability might more properly be termed that the ability of the eye to ‘discriminate in time the interaction of single quantum events’. As I have shown the retina is capable of logic function in the picosecond time domain. This opens up many new avenues of thought.

For example, I believe that analysis will show that at normal light levels entering the pupillarily (?) constricted eye the individual devices of the retina will be able to separate individual quantum events. I am working on this.

END FOR NOW

This entry was posted on Thursday, September 7th, 2006 at 11:00 am and is filed under Running Commentary . You can follow any responses to this entry through the RSS 2.0 feed. You can leave a comment, or trackback from your own site.